Problem: $-qr + 7qs - 5q + 9 = -3r + 4$ Solve for $q$.
Combine constant terms on the right. $-qr + 7qs - 5q + {9} = -3r + {4}$ $-qr + 7qs - 5q = -3r - {5}$ Notice that all the terms on the left-hand side of the equation have $q$ in them. $-1{q}r + 7{q}s - 5{q} = -3r - 5$ Factor out the $q$ ${q} \cdot \left( -r + 7s - 5 \right) = -3r - 5$ Isolate the $q$ $q \cdot \left( -{r + 7s - 5} \right) = -3r - 5$ $q = \dfrac{ -3r - 5 }{ -{r + 7s - 5} }$ We can simplify this by multiplying the top and bottom by $-1$. $q= \dfrac{3r + 5}{r - 7s + 5}$